1. Field of the Invention
This invention relates to iterative decoding of signals in a Multiuser Detection and Demodulation System (MDDS), e.g. a base station in a digital wireless communication system.
2. Description of the Related Art
Recent improvements in communication theory (using the "Turbo Principle") have shown that in certain digital communication systems, involving a plurality of users in wireless communication with a receiver, an improvement in the quality of the decoded signal can be achieved by applying iterative decoding steps to the received data. In particular, "Iterative Equalization and Decoding in Mobile Communication Systems" by Bauch, Khorram and Hagenauer, EPMCC '97, pp 307-312, October 1997, Bonn, Germany, discusses the application of the Turbo principle to iterative decoding of coded data transmitted over a mobile radio channel. In order to be suitable for iterative decoding, a transmitted signal must be encoded by at least two concatenated codes, either serially or parallely concatenated.
The signal is then decoded in a first decoding step, a first decoder being used to decode the inner code and a second decoder is used to decode the outer code, to provide soft output values on the reliability of the hard decision values. In a first iteration the decoding step is repeated and the soft output values are used as input values for the first and second decoder.
In the particular application of a mobile communication system, the channel encoder and the intersymbol interference (ISI)-channel may be viewed as a serially concatenated encoding scheme, with the channel encoder acting as the outer encoder and the ISI-channel acting as an inner, rate 1, block encoder. Thus, iterative decoding is suitable for application, in particular, to the European Wireless Digital Cellular Standard "GSM" with the equalizer performing the inner decoding step and the decoder providing the outer decoding step. Where bad communication channel conditions (low SNR, fading, multipath propagation, etc) exist, an improvement in the Bit Error Rate (BER) may be achieved with each iterative decoding step until a BER floor is reached. The signal received by a base station is equalized to provide soft decision values of the received coded bits.
Repeating the decoding step several times can improve the BER of the received signal. However, each iterative decoding step consumes resources, such as memory, computation time and ties up dedicated ASICs (Application Specific Integrated Circuits). In a practical base station, the number of signals that can be decoded in parallel is limited by the number of signal processing units (SPU) available; providing iterative decoding hardware, such as digital signal processors (DSPs), and software to each SPU adds considerably to the cost and complexity of the base station.